The disclosure relates generally to optical coupling between a planar optical waveguide and an optical fiber, and particularly to optical coupling between a planar optical waveguide on a transparent substrate and an angle cleaved optical fiber using a grating coupler where the optical fiber axis is generally parallel to the substrate plane.
Low-cost optical links for short distance applications (<1 km) often employ multimode optical fiber for relaxed alignment tolerances to sources and detectors. The large size of the multimode fiber core makes the optical interconnections highly tolerant of lateral, angular and axial misalignments with respect to commonly used Fabry-Perot or VCSEL laser sources and broad area detectors. This approach is well-suited for low to moderate data rate applications where link signaling is carried out using a single optical wavelength.
When higher link bandwidth performance is required, it is desirable to multiplex multiple signals on a single optical fiber using WDM (wavelength division multiplexing). This approach is appropriate both in short link applications to avoid length-bandwidth limitations inherent to multimode fibers, as well as in longer link applications that employ single mode fiber. In these cases, wavelength multiplex (MUX) and wavelength demultiplex (DEMUX) operations are generally required at source and receiver ends of the link, respectively.
Planar lightwave circuits (PLCs) are well-suited for carrying out optical MUX and DEMUX operations. High index contrast waveguides enable implementation of tight waveguide bends with low optical loss. This allows fabrication of compact MUX and DEMUX devices on small chips, reducing chip cost and reducing overall package size. For example, MUX and DEMUX operations may be carried out using IIR (Infinite Impulse Response) ring resonator filter devices that are easily tuned to operate on specific wavelengths. Other devices, such as switches, modulators and power splitters may also be implemented using PLCs, extending the platform to other link and network applications.
While it is relatively straightforward to butt couple a multimode fiber to a high index contrast PLC waveguide at the chip edge, this interconnection is not always practical or desirable because it requires the coupling to occur at the edge of the chip, which creates a constraint on the layout of the PLC, and it places limitations on making the chip hermetically sealed. The lensed multimode fibers and single-mode fibers generally used in this application require precision alignment for low-loss coupling to PLC waveguides.
One approach for fiber coupling into PLC waveguides involves grating coupler devices. The grating coupler is typically implemented a broad taper shape to capture light incident on the coupler from an optical fiber and guide it along a PLC waveguide. The optical fiber is typically arranged to be generally normal or perpendicular to the PLC substrate. The same device can operate in reverse to launch light from a PLC waveguide into an optical fiber.